Issue link: https://iconnect007.uberflip.com/i/1284035
SEPTEMBER 2020 I SMT007 MAGAZINE 23 each subsequent step adds another opportu- nity to screw it up. The CAF test is used on bare fabs to determine whether process chem- istries are present on the inner layer of the PCB that will produce electrochemical migra- tion. This is the same as dendrite growth found on a fully populated PCBA. No matter where it occurs, if you have conductive resi- due, moisture, and potential, you run an ele- vated risk for electrical leakage and dendrite growth. The CAF condition is greatly aided by poor resin flow, creating dry weave that will absorb plating chemistries and allow them to bridge anode and cathode. The IPC test for CAF is found in TM-650, 2.6.25. This is an environ- mental test that is normally done on test cou- pons manufactured by your PCB supplier using the same materials you plan to use for nor- mal production. The test boards are subjected to elevated heat and humidity for at least 596 hours under bias. In Figures 1 and 2, you can see dry weave facilitating CAF that will render any subsequent processing steps meaningless. Scanning electron microscopy (SEM) and energy-dispersive X-ray spectroscopy (EDS) are among the best analytical tools for investi- gating CAF. By using SEM/EDS, you can deter- mine the composition of the material and com- pare it to the base metals being used for barrel plating. Figures 3 and 4 show SEM and EDS examples. If it matches, you have CAF and need to work with your PCB fabrication sup- plier to optimize the process. In your effort to optimize the bare fab pro- cess, you need to know what the contamina- Figure 1: PCB inner layer dry fiberglass weave (All photos taken at Foresite). Figure 3: EDS analysis of CAF. Figure 2: Example of CAF. Figure 4: EDS results of CAF.